It is often desired to use an EPDM rubber (ethylene/propylene/non-conjugated diene-based rubber) for various tire components, such as a sidewall component, primarily because it typically has superior resistance to ozone degradation. However, EPDM elastomers are of limited usefulness for such purposes, primarily because of their typically low resistance to cut growth propagation.
The problem is compounded where it may be desired to provide a tire component of a non-black colored rubber composition so that, therefore, carbon black is not suitable for the rubber reinforcement.
Accordingly, it is desired to prepare a tire component which has the desirable age resistant properties of EPDM yet with acceptable cut growth resistance, particularly for EPDM-based rubber compositions which do not contain carbon black reinforcement.
In practice, it is recognized that precipitated silica typically has only a limited ability for reinforcing elastomers, as compared to carbon black, without a presence of a silica coupling agent such as, for example, an organosilane polysulfide which contains at least about 3.5 sulfur atoms in its polysulfidic bridge
While, in practice, it is recognized that precipitated silica, in combination with an organosilane disulfide, has been used for reinforcement of diene-based elastomers for use as tire components, it is usually accompanied by adding free sulfur, or at least additional free sulfur, at some place in the rubber composition formulation. It is believed that this is well known to those having skill in such art.
It is to be appreciated that an organosilane disulfide, with its relatively strong sulfur-to-sulfur bonds, is not a good sulfur donor for rubber processing purposes, particularly as compared to an organosilane polysulfide which has at least 3.5 sulfur atoms in its polysulfidic bridge.
Accordingly, by itself, an organosilane disulfide would not ordinarily be considered a silica coupling agent for coupling a precipitated silica to a diene-based elastomer.
Further, a utility of an organosilane disulfide would considered to be very limited, if at all, in an EPDM rubber composition because of (1) its relative ineffectiveness as a sulfur source for coupling to carbon-to-carbon double bonds of an elastomer and, further (2) the relatively small amount of carbon-to-carbon double bonds available in an EPDM elastomer.
In practice, it is sometimes desired to color a portion of a sidewall to add some color other than black, albeit primarily for cosmetic reasons.
An inherent difficulty in providing such colored components for a tire is providing a non-carbon reinforced rubber composition with suitable properties for a tread sidewall.
In the description of this invention, the term "phr" is used to specify amounts of materials or ingredients in a rubber, or elastomer, composition in terms of parts by weight of such ingredient per 100 parts by weight of elastomer(s).
Also, in the description of this invention, the terms "rubber" and "elastomer" may be used interchangeably as well as rubber composition and rubber compound unless otherwise indicated.